Catalysts used in hydrocarbon conversion processes such as reforming, dehydrocyclization cracking, isomerization, alkylation, etc., normally become deactivated after a period of catalytic use. It is known to regenerate such catalysts by treating them with an oxygen-containing gas to burn off deactivating substances such as carbon in the form of coke. Regeneration of hydrocarbon conversion catalysts with oxygen has often been performed in situ, i.e., by leaving a bed of catalyst in the reactor in which it had been used for catalytic conversion and passing an oxygen-containing regeneration gas into the reactor and through the catalyst bed to burn the deactivating substances off the catalyst.
Many hydrocarbon conversion systems, particularly naphtha hydroreforming units, employ a process in which the hydrocarbon feed passes in series through two or more separate reactors. Each of the reactors contains at least one bed of catalyst, and each reactor is connected to one or more other vessels by large-sized conduits through which the feed is passed. When the hydrocarbon conversion reaction, or reactions, being carried out in a particular system are endothermic, as in reforming, a furnace is normally connected into each of the large feed conduits upstream of each reactor to add required heat to the feed.
In general, hydrocarbon reforming and similar hydrocarbon conversion systems are classified as (1) cyclic, (2) continuous, or (3) semi-continuous (i.e. semi-regenerative). Such classification is based primarily on the different catalyst regeneration methods employed in these systems. In cyclic reforming one of the series of reactors is taken off stream and isolated by means of high temperature valves so that the catalyst therein may be regenerated separately. In continuous reforming, portions of the catalyst are removed from the plant vessels, regenerated in a separate vessel and then returned to the reaction system. During semi-continuous reforming operations, catalyst in all vessels of the entire system is periodically regenerated by passing oxygen-containing gas in series flow through all reactor vessels. The present invention is primarily directed to regeneration of catalyst in a semi-continuous system, i.e. semi-regenerative reforming, or in situ regeneration, although the principles of sulfur removal from feed lines and heat vessels before coke removal finds application wherever oxygen-containing gas passes through plant internals containing metal sulfides.
During in situ regeneration of a catalyst in plural-reactor-vessel conversion units of a semi-regenerative reformer system, an oxygen-containing gas is conventionally passed in series flow through all the reactor vessels. The large feed conduits running between the reactors are used to pass the oxygen-containing gas from one reactor vessel to the next. Such flow is also through any heaters or heat exchangers connected in series with the reactors.
Hydrocarbons used as charge stocks for hydrocarbon conversion systems often contain some sulfur compounds. During the on-stream operation of a conversion unit, sulfur in the feed reacts with iron on the surface or in the linings of the conduits, the reactor, furnaces and other vessels, to form iron-sulfide scale. Most of the sulfur reacts with plant internals upstream from the first catalyst bed. To some extent, sulfur may also be deposited directly in catalyst beds during hydrocarbon processing. In semi-continuous catalytic reforming units, sulfur contamination is usually a problem primarily in the furnaces and heat exchangers and interconnecting conduits employed to heat the feed being charged to the first stage reactor.
When a catalyst is regenerated in situ in a unit which has been contaminated with sulfur, oxygen in the regenerating gas reacts with sulfide scale to form sulfur dioxide. Some hydrocarbon conversion catalysts, e.g., those containing platinum may in turn catalyze the reaction of sulfur dioxide and oxygen in the regeneration gas, at certain temperatures, to form sulfur trioxide. Sulfur trioxide may then react with alumina in the catalyst to form a sulfate, displacing halide and partially displacing catalytic metals, such as rhenium, and to a lesser extent, platinum. The presence of the sulfate prevents a halide, such as a chloride, from reacting with the alumina surface and thereby impedes redispersion of catalytic metals on the alumina to a form in which the catalyst metals have a highly exposed surface area. Since an effective redispersion of catalytic metals over the alumina surface is essential for proper catalyst regeneration, the presence of sulfates on the catalyst surface during halide addition is undesirable. Moreover, sufficiently high concentrations of sulfate can displace rhenium from the alumina, thereby removing the catalytic benefits that the presence of rhenium imparts to a platinum-on-alumina reforming catalyst. Where the hydrocarbon conversion catalyst is primarily a zeolite having a surface that is basic, rather than acidic, such conversion of sulfur dioxide to sulfur trioxide in the presence of the catalysts is even more deleterious to the proper catalyst regeneration because of its interaction with the base; upon subsequent reduction during start-up, hydrogen sulfide is released and reacts with the platinum and poisons the catalyst.
Chemical cleaning of the whole conversion system effectively removes sulfur from all the vessels, and thus prevents substantial sulfate contamination of catalyst beds, but is costly and time-consuming. The present invention provides an economical and easily performed process for reducing formation of contaminant sulfate in the catalyst bed when the reactor containing the bed is connected to other vessels such as furnaces and reactors by large feed conduits. The present process may also prevent sulfur contamination of downstream catalyst beds, and heaters during successive regenerations and on-stream periods following a run in which severe sulfur contamination occurs.
The reasons such downstream contamination of catalyst beds (other than the first bed) occurs is as follows: When a reformer not previously contaminated by large amounts of sulfur is exposed to a major sulfur upset, most of the sulfur tends to react with the internal surfaces of the plant (e.g., feed/effluent heat exchanger, feed furnace) upstream from the first reactor. During the next conventional regeneration, sulfur in the form of metal sulfide on said internal surfaces of the plant is converted into SO.sub.2 and flows into the first reactor together with O.sub.2. The SO.sub.2 is oxidized over the platinum catalyst to SO.sub.3, which displaces chloride and rhenium from first reactor catalyst. During the next on stream period, sulfate in the contaminated catalyst is reduced to H.sub.2 S, which tends to react with hot iron plant internals, especially the next, or first inter-reactor, furnace. During the next regeneration, sulfide in this furnace is oxidized to SO.sub.2 and subsequently to SO.sub.3 which displaces chloride and rhenium from the second reactor catalyst, etc.
The present invention is primarily effective for removal of sulfur deposited upstream from the first reactor catalyst. Reaction with the coke on first reactor catalyst consumes the O.sub.2 and prevents it from reaching downstream iron surfaces, such as interreactor furnaces. The present regeneration process is also applicable where as in application Ser. Nos. 344,570 and 344,572, both filed Feb. 1, 1982, assigned to the assignee of the present invention, the catalyst or a portion thereof is a zeolite catalyst having a group VIII metal component, such as platinum and an alkali or alkaline earth metal component, such as barium, strontium, or calcium as a dehydrocyclization enhancing material. The present regeneration process also is useful where such a zeolite catalyst is employed for dehydrocyclization.
It has previously been suggested, as in U.S. Pat. No. 3,137,646 Capsuto, to isolate various heavily sulfur-contaminated elements of a hydrocarbon conversion unit, such as furnace and heat exchanger tubes, before contacting a deactivated catalyst with an oxygen-containing regeneration gas and to purge iron sulfide from the isolated heat exchanger tubes with high-temperature steam and/or an oxygen-containing gas. The freed particles of sulfide scale and/or sulfur dioxide-containing gas are then removed from the system. This type of vessel cleaning requires that several valves or similar apparatus be installed directly into large feed conduits. Such apparatus is expensive and is unnecessary for normal operation of the conversion system and is used solely in relatively infrequent regenerations of the catalyst. This procedure also requires unusually high-temperature conditions.
Other procedures for eliminating sulfide scale from heaters and other sulfur-contaminatable vessels in hydrocarbon conversion units are deficient in that they fail to provide an effective method of preventing deposition of sulfur, as the sulfate, in a catalyst bed during in situ catalyst regeneration, which results in hindering proper redispersion of catalytic metals on an alumina catalyst base.
U.S. Pat. No. 3,716,477 Jacobson et al, assigned to the assignee of the present invention, is generally directed to a reforming operation for naphtha feed through a catalyst comprising platinum and rhenium supported on a porous inorganic oxide carrier in the manufacture of high octane gasoline. It discloses a regeneration process for maintaining catalyst activity which includes passing a gas containing nitrogen and oxygen through the catalyst bed at a temperature that is from about 750.degree. to 1200.degree. F. by maintaining the oxygen content in an amount less than about one volume percent. The patent does not disclose or discuss the problem of sulfur dioxide or sulfur trioxide contamination of the reforming catalyst during subsequent halide addition at high temperatures.
U.S. Pat. No. 4,155,836 Collins et al, is directed to a reforming system using a catalyst which includes a platinum group metal and a halogen. The regeneration process involves decontamination or regeneration of the catalyst when it has been directly contacted by a high sulfur and water content hydrocarbon feed stock. The process includes discontinuing feeding of the contaminating feedstock and passing hydrogen over the contaminated catalyst. Reduction of a platinum on alumina catalyst with hydrogen results in the formation of H.sub.2 S in the effluent gas from the catalytic reactor. In the case of a semi-continuous catalytic reformer, this results in sulfiding of the inter-reactor furnace between the first and second reactors. This sulfur can then be transferred to the second reactor catalyst during the next regeneration. This patent does not disclose any process for removing sulfur from the furnace tubes and heat exchangers during regeneration of the catalyst by heating and passing an oxygen containing gas through the furnace tubes and then through the reactor.
U.S. Pat. No. 4,033,898 Jacobson et al, also assigned to the assignee of the present invention, discloses a regeneration system wherein carbonaceous deposits on the catalyst are burned and sulfur is converted to sulfur trioxide which forms sulfates in the catalyst; thereafter the sulfate is removed from the catalyst by hydrogen treatment and the resulting reduced gases are removed.
U.S. Pat. No. 3,773,686 Hayes is directed to a multi-step catalyst regeneration process in which during each step it is essential that the gas employed in the regeneration be free of sulfur compounds, and in particular oxides of sulfur and H.sub.2 S. The method also contemplates the use of water in the conventional burning steps for catalyst reactivation.
To overcome some of the difficulties attendant upon the deposition of sulfide scale on and in the upper portion of a static bed of platinum-group metal catalyst U.S. Pat. No. 2,884,372 Bergstrom discloses submerging foraminous baskets in the upper portion of the static bed of platinum catalyst.
U.S. Pat. No. 2,792,337 Engel shows that oxygen-containing gas may be introduced into the catalyst bed without prior contact with parts of the reactor and feed inlet line and to pass part of the gas back through the forepart of the catalyst bed and the feed inlet line of the reactor and the remainder forward through the catalyst bed without recycling any part of the gas through the catalyst bed.
U.S. Pat. No. 2,873,176 Hengstebeck discloses that difficulties can be avoided by not exposing the sulfide scale in the heater tubes to free oxygen. Hengstebeck teaches to pass inert carrier gas through the heater and to inject sufficient oxygen to produce combustion of the carbonaceous material, usually designated as coke, into the carrier gas between the heater and the reactor.
U.S. Pat. No. 2,923,679 Thomson teaches that the heated oxygen-containing regeneration gases flow through the reforming unit in a direction which is the reverse of the flow of naphtha and hydrogen-containing gas.
As contrasted to other systems, the present invention does not depend upon diverting, through additional expensive valves and auxiliary conduits, the sulfur-containing gases resulting from regenerating the catalyst and auxiliary equipment. To desulfurize both auxiliary equipment and catalyst during regeneration of the catalyst, this invention is directed to an improved regeneration method requiring only control of temperature, pressure, rate of flow, and oxygen-content of gas flowing through the reforming unit, including furnaces, connecting conduits and catalyst-containing reactors, first to remove sulfur contaminants from the system and then to remove carbon components from the catalyst before redispersion of platinum and/or rhenium components on the catalyst at high temperatures, with or without addition of halide to the regenerating gases.